It's not an easy fix, but there's been a lot of scholarly work that points to some of the "societal beliefs" you talk about reverse engineering (grad school protip: that reverse engineering is called "deconstructionism" and there's an entire discipline called "gender and women studies" that delves into it ;)). I haven't scanned all the comments yet but I suspect a good number of them call for things like "education" or "coaching" on negotiation -- basically, that "women should ask for more". The thing these solutions overlook is that the root of the problem is socialized, enforced gender roles and there are serious social consequences for women who overstep these boundaries.
An interesting recent study study studied the behavior of women when negotiating for themselves vs negotiating for others. When negotiating for themselves, they exhibited the kinds of behavior in the original link, ending up with salaries far below the men in the study. When they were asked to negotiate for someone else, however, they performed on par with the men. (link: http://www4.gsb.columbia.edu/news/item/7223326/Role+of+Gende..., original: http://psycnet.apa.org/journals/psp/98/2/256/) And you know that oft-cited study that men think about sex way more than women? Well, they also think about sleep and food at about the same rate.. and way more than women. One way to interpret these results is that women are socialized to think less of their own needs (and to voice them less) -- and this is a societal norm that's heavily enforced with consequences for those who don't toe the line.
Think about how strong women are regarded vs men who exhibit the same behavior. A woman that does what she needs to get what she deserves runs the risk of being called a "bitch" or, at the very least, being thought of as unfeminine. Personally, I find fucking with gender norms an amusing hobby so I don't give a shit about the consequences.. but a lot of women aren't into that ;) Just remember this when thinking about possible solutions -- the root causes are way deeper than the appallingly patronizing "women need to be taught how to negotiate."